The present invention relates to a method of bonding copper and a resin together, and, more particularly, to a method of bonding copper and a resin together with consistently high bonding strength and reduced formation of pink rings and laminate voids on a printed circuit board by treatment of the oxide layer formed between the copper and the resin.
Since no sufficient bonding strength between a metal and a resin can be secured by directly bonding the resin onto the smooth surface of the metal, a known method has heretofore been employed in which an oxide layer is formed on the surface of a metal to improve the bonding strength, as described in "Plating and Surface Finishing" vol. 69, No. 6, pp. 96-99 (June, 1982).
Nowadays, the formation of a layer of copper oxide on an inner surface of copper so as to enhance the bondage between copper and resin has become an inevitable step in the production of printed circuit boards or multilayer printed wiring boards. However, an inherent problem in the method is that copper oxide is readily eroded upon contact with an aqueous acidic solution by becoming copper ions dissolved therein. In addition, the brittle and poor mechanical properties of the oxide layer as well as the tendency of the oxide layer to get wet and contaminated easily have, nevertheless, caused "pink rings" and "laminate voids" on boards thus produced. The formation of pink rings and laminate voids represents serious defects in the boards especially in an era when increasingly high quality and reliability are demanded. These defects have become a bottleneck in the production of multilayer printed wiring boards to pursue further and higher development.
U.S. Pat. No. 4,642,161 discloses a method of 10 bonding copper and a resin together which comprises the steps of forming a copper oxide layer on the surface of copper by oxidation of copper, reducing the copper oxide layer to metallic copper with a reducing solution, and bonding the surface of the metallic copper formed by the reduction and a resin together. However, formaldehyde cannot be used as the reducing solution since the copper oxide layer formed on the surface of copper is hardly reduced to metallic copper with formaldehyde under usual conditions.
Nevertheless, formaldehyde is one of the best reducing agents because it possesses the characteristics of low cost, easy sewage treatment, and high stability.
U.S. Pat. No. 5,006,200 discloses a method of bonding copper and resin which comprises the steps of forming a layer of copper oxide on a surface of copper by oxidation of copper, reducing the layer of copper oxide thus formed to cuprous oxide with a reducing solution, and bonding the surface of the cuprous oxide formed by the reduction and a resin together by heat-pressing.
To enhance the bonding strength between copper and resin, as is well-known, a layer of copper oxide was formed between the copper and resin in the production of printed circuit boards or multilayer printed wiring boards. The formation of the copper oxide is:
a) to act as an interface between the copper and resin to prevent the direct contact of resin with copper which may cause premature aging of the resin; and
b) to provide a surface of higher roughness so as to enhance the bonding strength between copper and resin.
However, an oxide layer is very fine in structure and is subjected to contamination by dirt and humidity which cause the formation of "laminate voids". Furthermore, as the oxide layer is easily eroded to dissolve upon contact with an acid, the area where the oxide layer has been eroded reveals the color of the underlying copper which is pink in color and thus causes the formation of the so called "pink rings".
The area where the formation of "laminate voids" and/or "pink rings" occurs indicates that the resin is not in contact with the copper and, thus, that there is poor bondage therebetween. In other words, the greater the number of and the larger the size of the presence of "laminate voids" and/or "pink rings" on the printed circuit board, the lower the quality of the printed circuit board.
Unfortunately, nearly all printed circuit boards with more than four layers suffer from the formation of "laminate voids" and/or "pink rings". Therefore, the reduction in number and size of the "laminate voids" and "pink rings" has become one of the most popular and important topics in the field of the production of printed circuit boards and multilayer printed wiring boards.
Accordingly, a novel method of bonding copper and resin which mitigates and/or obviates the above-mentioned drawbacks is provided.